In the manufacture of telecommunications cable, sheathing for the cable is commonly produced by forcing plastic material through a heated extruder to emerge as a filamentary body of insulation. During such procedure, the extruder occasionally becomes hot enough to produce at the body's surface a defect in the form of a localized nodular region in which the insulation has become partly carbonized to appear as a "black speck". Within that region the conductivity value of the insulation for electric current is, because of its carbonization, much greater than for the normal insulation.
If the cable sheathing has thereon one or more of such specks which are not detected so that they are not removed or otherwise compensated for, the continued presence of such one or more specks in the finished cable may produce signal losses thereon or other detrimental effects in the use of the cable as a medium for conveying information.